Reprocessing of either spent nuclear fuel or weapons material results in liquid waste which must be reduced in volume and consolidated to permit safe disposal. The current practice is to dehydrate the liquid waste by heating, then to consolidate the residue by either calcination or vitrification at high temperatures. In the past, defense waste was neutralized in order to precipitate metallic hydroxides. This product can be converted into a vitreous waste form using conventional glass forming technology.
The ultimate suitability of vitreous waste forms is suggested by the durability of rhyolytic obsidian and tektite natural glasses during millions of years in a variety of geologic environments. Unfortunately, these chemically durable, high-silica glasses pose problems as a practical solid-waste form, when made using conventional continuous vitrification processes. Because of the high fluxing temperatures (.about.1350.degree. C.) required, additional off-gassing scrubbing capacity or other absorbent procedures are needed to deal with the volatilization losses of radionuclides such as iodine, cesium, and ruthenium. The high fluxing temperature also shorten furnace life, and can create problems with the materials into which the molten glass is cast, such as the sensitization of stainless steel to stress corrosion cracking. As a consequence of these limitations, most nuclear waste glass formulations have substantially lower silica content than either natural obsidians, nepheline syenite, or commercial "Pyrex" glasses. Less silica or alumina and more fluxing agent (e.g., Na.sub.2 O, K.sub.2 O or B.sub.2 O.sub.3) lowers the glass working temperature (to 1000.degree.-1200.degree. C. for most waste glasses) and raises the waste loading capacity. However, this also results in lower chemical durability in most aqueous environments and, particularly for borosilicate compositions, in less resistance to devitrification.